Soil Management: Soil Fertility Management

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Law and Tan (1973) and Goh et al . (1993) had clearly shown that large variations in soil fertility occurred within and between soil series in Malaysia. These variations also occurred spatially at macro and micro scales (Goh et al ., 1995) thus demanding site-specific management approach to maximise efficiency. This is in agreement with the varied responses of plantation tree crops to fertilisations, where yield responses for oil palms ranged from 0 to 250%, for cocoa from 0 to 47% and for rubber from 0 to 39%. This further indicates that soil fertility management is not only site-specific but also crop-specific.

The realisation of above has caused scientists to develop schemes or methods to measure or assess soil fertility quantitatively or qualitatively. One of the schemes called fertility capability classification system (FCC) has been discussed in earlier lecture. In plantation tree crops, the assessment of soil fertility generally takes the format of single nutrients approach as shown in Appendix 4. The soil physical and biological properties are not included because they are generally handled separately.

This part of the lecture will cover only the fertiliser management of soil fertility since the other aspects have been dealt with.

Nutrient requirements

The nutrient requirements of plantation tree crops are usually calculated based on the nutrient balance concept (Chew et al ., 1994b; Kee et al ., 1994). This involves the equating of factors of nutrient removal against those of nutrient supply (Figure 1). Therefore, the fertiliser requirements will depend, apart from the crop removal, also on the inherent soil nutrient status. Foliar analysis is also used as a supplementary tools for the diagnosis of nutrient requirements.

Therefore, the key steps of an effective fertiliser management programme are :

• determination of growth and yield targets,
• assessment of the action required;
  1. What nutrients are needed?
  2. What rates of nutrients are needed?
  3. How best to achieve the most efficient and cost effective application of fertilisers to meet nutrient requirements ?
  4. What types of fertiliser to apply?
• Assessment of the results and further action required,
• Computation of the economics of the results.

Detailed description of the above is provided by Chew et al . (1994b) and interested readers should refer to the paper. We shall instead discuss the fertiliser application technique in plantation tree crops which is one of the key factors in determining an efficient and environment friendly approach to soil fertility management.

Fertiliser application techniques

The higher the fertiliser efficiency the lower is the risk of manuring on the environment. This simple relationship demands that we maximise or attain satisfactory efficiency of the fertiliser applied. Proper application methods are essential to achieve this, especially in areas where the soils are proned to high run-off and leaching losses and to combat these, we generally rely on frequency, timing and placement of applied fertilisers.

Frequency of application

Foong (1993) using field lysimeter reported that after the first four years, low leaching losses in Munchong series soil were recorded for all nutrients except Mg (Table 6). However, Chang and Zakaria (1986) working on the sandier Serdang series recorded leaching losses of 10.4% for N and 5.1% for K with 2352 mm of rain per year. This apparent correlation between nutrient loss via leaching with soil texture was also illustrated by Pushparajah et al . (1993) in laboratory trials.

Time of application

Although we are in the humid tropics, the rainfall patterns differ considerably between locations. On-going studies (Chew et al ., 1994b) show that high rainfalls prior to fertiliser application resulted in substantial nutrient loss, especially in high fertiliser concentration areas (Table 7).

The general guideline is to avoid fertiliser applications during:

• period with high rainfall months of more than 250 mm month^-1,
• months with high rainfall days of more than 16 days month^-1,
• months with high rainfall intensity of more than 25 mm day^-1.

Placement of fertiliser

Fertilisers should be applied in areas with anticipated active root development and maximum feeder root distribution, which vary according to plant age and species. Therefore, fertilisers are applied close to the tree base in the initial years and gradually extended to the tree avenues when the canopy has overlapped and good root development is found there. In hilly terraced areas with mature trees, the fertilisers should be applied broadcast in the terrace itself and between the trees. In areas with platforms, the fertilisers should logically be placed around them.

Application of palm oil mill by-products

The palm oil mill produces substantial amounts of by-products such as EFB and anaerobic sludge. The applications of these by-products are encouraged because they return the organic matter and nutrients to the soil and hence, help to maintain soil fertility without causing environmental pollution.

In rubber and cocoa systems, the by-products are seldom returned to the fields due to their low nutrient values, logistic problems and maintenance problems. It has to be noted that pod husks in cocoa plantations are thrown back to the field during harvesting.

EFB

Gurmit et al . (1982) reported that 1 tonne of EFB contains 15.3 kg of ammonium sulphate, 2.5 kg of Christmas Island rock phosphate (CRIP), 18.8 kg of muriate of potash and 4.7 kg of kieserite. Hence, in mature oil palms, 40 t ha^-1 of EFB are generally applied in the interrows to supply sufficient nutrients for a year. Supplementary fertiliser applications such as CIRP may be required to balance the nutrient requirements of oil palms.

Apart from being a source of nutrients, EFB also improves the soil physical properties and reduces soil water evaporation (Lim and Messchalck, 1979). Therefore, preference for EFB application should be given to problem soil areas such as the sandy podzols and shallow Malacca series.

Anaerobic sludge

The application of anaerobic sludge would also help to partly relieve moisture stress in soils susceptible to moisture deficits, in view of the considerable amount of water in the sludge. The usual recommended rate of application for mature oil palms is 450 l palm^-1 yr^-1. The fertiliser equivalents according to the nutrient composition of 3.6g N, 2.4g K, 1.2g P and 1.5g Mg per litre (Lim, 1984) are 7.6 kg palm^-1 of ammonium sulphate, 1.6 kg palm^-1 of CIRP, 2.1 kg palm^-1 of muriate of potash and 2.6 kg palm^-1 of kieserite.

Supplementary fertiliser applications may again be required to ensure balance nutrition. The application areas of anaerobic sludge should also be in the palm avenues.